Instrument navigation system



Aug. 26, 94?. R. c. HoPGooD, JR

A INSTRUMENT NAVIGATION SYSTEM 3 Sheets-Sheet l Filed March 25, 1942 14N TEN/VAE OR/NTABLE n F 1 N 4 w Ew am W0 N Hv/\ I6. 0l H4 0 Y M H 4 Y y n V Y K R R K B n. E E YR. Y0 7 0,/ Aw w R 4 mw .a r u fr M M w W a M o T M vm m Z ,M FM @M wn 5 u. Z sa uw MP6 wwn., o V m. .n R R s e e we www. mm 03M .m me pm wf i F i W@ ,gam kw ma .mn mm m 6 E WW vDuim MA 0.A f d 0 R W ,w w a w m s ,9 N /1 .r A. v 6 m ATTRMY Aug 25, 39?. R. c. HoPGooD, JR ,426,2

INSTRUMENT NAVIGATION SYSTEM Filed March 23, 1942 3 Sheets-Sheet 2 6e 952 y J2 aLl/PERI /61 if 0N COURSE PAST @UTEP APPROACH/NG FPDM /N G4 IDE PLA/YE REPA/A TRS WRONG DIRECT/0N LEFT 0F Gal/RSE /N 5L/DE PLANE /N GLIDE PLA/VE /N GL/OE PLANE 0N COURSE 0N COURSE' INVENTOR.

oy C'. HopooDJ/P.

ATTORNEY Aug. 26, w47. I R. c. HoPGooD, JR 1 2,425,238

INSTRUMENT NAVIGATION SYSTEM Filed March 25, 1942 3 Sheets-Sheet 3 (a J jL Mh J' MMA/Vw WM' g TRA N6 M/ T TE l? b U/VBL UCK/IVG 226 H secr/@Af 5s T0/veo m 'Armi/m Patented ug. 26, 1947 U'ITED INSTRUMENT NAVIGATIN SYS'l-Elvl Application March 23, 1942, Serial No. 435,873

13 Claims.

This invention relates to radiant energy systems for aiding in the navigation of mobile bodies,. and particularly -to such systems as aid in the landing and. other navigation ofv aircraft.

It is a broad object of the invention to provide an improved navigation system.

It is also an object to provide an improved instrument landing system.

Another object is to provide an improved instrument landing system employing the transmission of brief trains of ultra-high-frequencyl waves whereby highly enicient functioning is obtained.

A further object is to provide a secret instrument landing system in whichA there is normally noradiation of energy from the vicinity of the desired landing area, whereby the landing area willnot 4be detectable by ordinary direction finder equipment and hence may not'be readily locatedl by enemy aircraft.

Still another object is'to provide an improved instrument landing system wherein apparatus to be installed. in the vicinity of the desired landing area may be extremely simple and readily portable.

It is also an object to provide an improved instrument navigation system wherein existingv obstacle-detection and/or absolute altitude apparatus on the aircraft may be employed without requiring additional equipment on the aircraft.

Av still' further objective is to provide an improved instrument landing system which is rela.- tively unaffected by changing weather conditions.

Other objects and various further features of novelty and invention will hereinafter be pointed out or will become apparent to those skilled in the art from a reading of the following specification in conjunction with the drawings included herewith.

In said drawings- Fig. 1 is a schematic showing in perspectiv of a landing field provided with a set of guiding equipment according to the invention;

Fig. 2 is a schematic circuit diagram of one known type of obstacle-detection apparatus which issuitable for employment on an aircraft for `cooperating with the landing field guiding equipment of Fig. 1 in accordance with the invention;

Figs. 3 and 3a are schematic circuit diagrams of alternate forms of guiding apparatus to be installed in the vicinity of ak desired landing arca. inv accordance with the invention;

Curves a; b,- c, d, n of Fig. 4 graphically 2 illustrate wave-forms'within th circuitof Fig. V31; and

Figs. 5 through 13- represent possiblel indications obtained with the apparatus of Fig'. VZ'und'er various conditions of flight in the neighborhoodr of a landing area equipped in accordance with the invention.

Broadly speaking, the inventionl contemplates the employment for the purpose of instrument landing of obstacle detector and locator apparatus normally carried on aircraft for other purposes. Many such detector systems have been devised such as those described', for example; in the kco-pending applications forpatent of- Messrs.- Labin and Busignies, Serial No. 383,108l filed? March 13, 1941; Serial No: 382,391 filed Marchf, 1941; and Serial No. 417,151 filed October 30, 1941'. In accordance with these Systemslal transmitter on theV aircraft intermittently radiates brief trains of ult-ra-high-frequency waves generally forward of the aircraft, and' directionallyresponsive antenna means are provided.l onjthe aircraft for picking up directional 'componentsof received reflections of the Wave trains sotransmitted. The reflected energy componentsy thus receivedare thereafter detected and appropriately combined in groups and` supp-lied: tothe control: system of a cathode ray oscillogr-aph device whereby a simultaneous indication. of the direction of and distance to' the reflecting Objectis graphically depicted on the screen otV the. oscillograph;

In accordance withthev inventionvrI. propose to. set up a plurality of reradiating.V or reflecting'.

members about a desired landing area. When a. pilot flying an aircraft equipped with theabovementioned apparatus arrives i'nthe vicinity` of.' the airport, hemay detect a plurality of indications in his oscillographl screen corresponding to.` this plurality of reradiators or` reflectors about the landing area. Knowing what kind ofi configuration of such indications to expect for: al proper glide path', the pilot may maneuver his craft safely above allA ground obstaclesuntil such a configuration is obtained. Thereafter all the pilot has to do is-to maintain substantially' the same configuration in a normal gldeand he will conduct himselfA` safely to the desiredlanding area.

The broad'l operation ofi al preferred? set-up in.

accordance with theI invention will' bebetter understood by reference toFig. 1... This schematic: showing depicts in perspective. the vicinity' oft a desired landing area I0 about which are-disposed' a plurality of rele'ctingunits Il, l2,A I3'.l Inord'er' to avoid any possibility of collision with reflectors II, I2 and I3, these units are preferably disposed outside of the desired landing area or under the ground level. In the form shown, reflector II is arranged along the axis of the desired landing direction I4 and generally centrally of landing area I0. In order to avoid any possibility of collision with reflector II, it is located under the ground in a pit covered with non-shielding material, such as planking in a well-known manner. Reflectors I2 and I3 are preferably spaced equally from the desired landing axis and ahead of the desired landing position, as shown.Y Reflectors I2 and I3 are also supported in raised position with respect to reflector II by means of appropriate poles or towers I5 and I6. The elevation of reflectors I2 and I3 with respect to reflector II is such that the three reflectors I l, I2 and I3 uniquely determine a plane making an angle with the plane of landing area I so as to determine an appropriate glide angle.

With the system thus far described it will be clear that all the pilot of an aircraft I'I equipped with obstacle-detection apparatus of the aboveindicated nature has to do to come down at the correct gliding angle is so to manoeuvre his plane that his obstacle-detection indicator discloses three objects directly ahead, all of them being in the Same plane and symmetrically disposed there- A still better understanding of operation of the apparatus in accordance with the invention may be obtained from the following disclosure of specific circuits which may be employed asthe obstacle-detection apparatus carried by the aircraft and as reflectors II, I2 and I3.

Fig. 2 schematically shows an obstacle-detection circuit suitable for installation on aircraft. This circuit is generally of the nature disclosed in the aboye-mentioned co-pending application of E. Labin. In accordance with this circuit, an impulse generator I8 supplies a series (which may be intermittent or continuous and periodically recurrent) of extremely sharp impulses to a transmitter I9. Transmitter I9 then supplies Ibrief wave trains corresponding to these impulses to a generally forward directional element 2Q of the antenna system 20 for radiation in a direction generally forward of the aircraft. Antenna system 2B further includes four directionally discriminating antenna elements 2l, 22, 23 and 24, all of which are generally directional in a forward sense but each of which is predominantly sensitive in one of four quadrants. Thus, antenna element 2| is predominantly sensitive upward and to the left of the aircraft in a forward direction, antenna element 22 is predominantly sensitive above and to the right of the aircraft in a forward direction, antenna element 23 is predominantly sensitive below and to the right of the aircraft and in a forward direction, and antenna element 24 is predominantly sensitive below and to the left of the aircraft and generally forward thereof.

Receiving antennae 2l, 22, 23. 2d are each connected to one of detector amplifier means 2l, 28, 29, 30, as shown. In order to prevent these receiving antennae from sending large signals to the receiving circuits of networks 21 through 3Q for each wave train transmitted from antenna 20, I preferably provide antenna blocking means 25 associated with each of the transmission lines connected to antennae 2I through 2li. Antenna blocking means 25 is preferably of the nature disclosed in the co-pending application of R. B. Hoffman, Serial No. 418,671, led November 12, 1941, and serves effectively to block these transmission lines at least during the instant in which a wave train is 'being transmitted. As disclosed in the said Hoffman application, circuits 25 are quarter-wave-length transmission lines, shorted at one end, and tuned or detuned in accordance with a control signal. Such circuits will hereinafter be referred to as coupled sections. When tuned and disposed closely adjacent the transmission line, a coupled section will effectively block transmission along the line, and when detuned, energy may pass along the line.

As indicated, control impulses for operating coupled sections 25 are preferably of a duration greater than that of the transmitted wave trains so as more positively to exclude any direct reception of the transmitted energy. Accordingly, a generator I8' of longer impulses than those supplied by generator !8 is shown synchronized therewith and coupled to the control `elements of coupled sections 25. In a preferred embodiment, impulse generator I8 is of the nature disclosed in the co-pending application of E. Labin, Serial No. 406,499, filed August 12, 1941, wherein impulses are generated by the intermittent charging and discharging of an inductance. In such a generator, considerable current must be passed through the irtfluctance for an interval prior to the discharge impulse, and, accordingly, a voltage proportional to this charge current is used to generate the longer impulses for controlling coupled sections 25. Since this charge current necessarily commences several microseconds prior to the discharge (which is of the order of two microseconds duration), ample time is afforded the control circuits for coupled sections 25 properly to resonate these sections so as assuredly to block the transmission lines associated therewith by the time a Wave-train is transmitted at the instigation of a discharge of the inductance of impulse generator I8.

Since a steady control potential isrequired to keep coupled sections 25 tuned to resonance, generator I 8' includes limiting means for this purpose. It is clear that by the time the inductance of generator I8 is discharged, the blocking impulse supplied by generator I8' is extinguished and coupled sections 25 are returned to their normal detuned state so as to permit passage of energy along the lines with which they are associated. In this manner, almost immediately following the transmission of a wave train, the receiving antenna circuits and receivers 21 through Sil are conditioned for reception whereby uni-directional impulses indicative of the relative strength of reflections picked up by the respective receiving antenna elements maybe obtained in the respective outputs of detector ampliers 2l through 30. Thus, the magnitude of uni-directional impulses obtained in the output of detector amplifier 2l will be indicative of the magnitude of reflection of energy in the lower right quadrant ahead of the aircraft, the magnitude of uni-directional impulses obtained in the output of detector amplifier 28 will be indicative of the magnitude of received reflections in the upper right quadrant and ahead of the aircraft (and similarly for the magnitudes of unidirectional impulses obtained in the outputs of detector amplifiers 29 and 3K1).

In accordance with the teachings of the abovementioned La-bin application Serial No. 383,108, each of the respective outputs of detector-amplifier circuits 2T through 3i] is employed for simultaneous application of directionally indicative a/ieazls deflection potentials to the deflection systems oi a cathode ray indicating device 3l. In the form shown, these deilecting potentials are obtained as follows: The outputs of detector-ampliiiers 28 and 29, representing the greater part of the energy received in a generally upward sense, are both applied to a. so-called up summation network 32, whereby a single indication of energy received in an upward sense is obtained. In the same manner the outputs of detector-ampliers 2l and 39, representative of energy received in a generally downward direction from antenna elements 23, and 24, may be applied to a so-called down summation network 33. In order then to obtain an indication of the relative upness or downness of a reflected signal, the outputs of networks 32 and 33 may be compared to each other, as by providing an inverter circuit'Sl to permit effective subtraction. In the circuit shown inverter network 3d is in series with the down summation network 33, and these two circuits combined may involve a greater delay in transmitting an impulse than does passage through the up summation network 32. In order, therefore, to permit an accurate superposition of inverted down summation pulses with respect to up summation pulses, appropriate delay means 35 is included in the circuit of the up summation network, thus assuring accurate timing. Pulses present in line 3S are thus clearly indicative of the relative upness or downness of the received reilected impulse energy and may be directly applied to the vertical deilection system 3l of cathode ray tube 3l.

In an analogous manner, the outputs of detector ampliers 2'! through 36 are also employed to apply delecting potentials indicative of the relative rightness or leftness or received reflections of impulse energy to the horizontal delection system d3 of cathode ray tube 3l. To this end, the respective outputs o1" detectorampliers 2l and 28, representing energy received by antenna elements 22 and 23 (in a direction generally to the right) are supplied to a right summation network S8, whereby pulses indicative of total energy received from the right are obtained. Similarly, outputs of detector amp-liers 29 and 36 are connected to a left summation network 39 in order to obtain pulses indicative of energy received in a leftward sense. As in the case of the up and down summation circuits 32 and 33, an inverter network 1353 and appropriate delay means @i are included in the respective circuits 3S and 39. The inverted and appropriately delayed outputs of networks 38 and 3Q are then superimposed in line 62; and, when connected to the horizontal deiection system 3, cathode ray tube 3l may be supplied with potentials indicative of the relative rightness or leftness of the received signals. referably the summation circuits 32, 33, 38 and 33 and the delay and inverter circuits lo and lli are so designed and adjusted that the total delay ror any particular wave train received by the antenna system 28 is exactly the same when applied in deflection potential form to deilecting systems 3l and i3 of the cathode ray tube. In this manner, it will be clear that, for any particular wave train received, one singleresultant deflection potential maybe applied to the deilection systems of the cathode ray tube, whereby for this particular received wave train there will be a corresponding directionally indicative deection of the cathode ray and hence of the image appearing on the screen of tube 3l.

In order to obtain an indication of the distance to the reflecting object, the circuit of. the above-mentioned Labin application provides a saw-tooth generator circuit @lll operating in synchronism with the generation of impulses, as shown schematically by a direct connection to generator 29. In such use of the word synchronism it is to be understood that a fresh saw-tooth wave is commenced for each impulse generated by network I8. The saw-tooth waves thus supplied by generator @Il are then applied directly to the vertical deection system 3T of cathode ray tube Si. In accordance with wellknown principles, the potentials thus supplied to the vertical deilection system serve to provide a vertical distance indicating axis on the screen or tube 3i so that, when directionally indicative deilection potentials are momentarily applied in the above-indicated manner to deiiection systems il and llt, their delay with respect to the transmitted impulses to which they correspond (which corresponds to the distance to the reilecting object) will be represented by their vertical position on this axis.

In order to avoid confusion'on the screen of 3i, I prefer to suppress any traces which l.iig .t be produced as a result of stray direct pick-up of emitted energy (or control energy involved in generating and transmitting such emitted energy). For this purpose, I block the ray of tube Si by supplying blocking pulses with impulse generator i8. These pulses serve to block out or suppress emission of the cathode ray in tube Si by applying -for the blocking interval (which is preferably the interval for which coupled sections 25 are tuned) a suiciently negative bias to a grid electrode l included in tube It will be clear that such a connection serves to prevent a visible recording on the screen of tube 3i indicative of transmitted wave trains.

Reduction of further undesirable visible indications cn the screen of tube 3i is accomplished by providing a normal negative bias, such as the battery dil', on grid de of suflicient magnitude normally to suppress emission of the cathode ray. In addition, a further network il is provided for permitting a reduction of the degree oi bias on grid it to a point above cut-od (whereby the cathode ray is produced) for the interval while impulse energy representative of received reflections of wave trains is being applied to the deflection systems Si' and d3. Such a network comprises amplifier di having its input coupled between network 32 and 35 and connected to apply deblocking impulses to grid 36. Amplifier 4'! is preferably of the limiting type so that the same strength cathode ray may be produced for all detected wave train reilections, as will be clear. Limiting amplifier fl'l is connected ahead or delay network 35, rst because the delay produced thereby is relatively slight, and also to permit the cathode ray to attain normal strength by the time deflection potentials are applied to divert the same.

With the system thus far described, it is apparent that for each received reflected wavetrain a single indication may be obtainedon the screen of tube 3l, and this indication may simultaneously indicate the distance and general direction of a reflecting object. Thus, in Fig. 5, which represents the screen of a cathode `ray tube having a vertical distance scale, indication i8 may meana reilecting object to the right of the aircraft carrying the apparatus. This indication will further mean that this reflecting object is about, say, one and a quarter distance units away and in the so-called horizontal plane of the detecting antenna equipment, (which may for the present be assumed to be in the plane of the wings of the aircraft). On the other hand, indication i9 may be indicative of another reflecting object to the left of the aircraft the same distance away and in this horizontal plane. The spot d appearing on the screen may indicate a still further reflecting object directly ahead and just short of two distance units away. The reason indication 5E! is a spot will be appreciated from the fact that, if an object is directly ahead, reflections therefrom as detected by antenna units 2l, 22, 23 and 2Q, respectively, will be of the same signal level, and, accordingly, deflection potentials applied to deflection systems 3l and i3 will be zero along either the vertical or horizontal axis.

With respect to indications @8, 4Q and 59 obtained on the cathode ray screen lshown in Fig. 5, it is torbe noted that all of the objects from which reflections are detected are at least in the abovementioned horizontal plane. The fact that indications i8 and d are substantially equal and equally spaced indicates a symmetrical disposition (of the objects indicated thereby) with respect to the axis of flight of the aircraft. Thus, the indications of Fig. 5 depict three reflecting bodies symmetrically disposed with respect to the aircraft and in the plane of its flight.

These indications lid, 19, 5@ convey information to the pilot that he is in the plane defined by the three reflecting antenna unts ll, i2 and i3 above discussed, and that the aXis of his antenna system 2c is directed at the far reflector l i. Once having obtained this set of indications, all the pilot has to do is to maintain the same configuration until the right and left horizontal indications i3 and 69 disappear off the screen of the cathode ray tube. When this occurs the pilot will be warned of the fact that he has just crossed a point near the approaching edge of the landing area and he may then probably see the ground and eect a safe landing. If by chance it is too foggy to see the ground once indications i3 and i9 have disappearedY from the screen of the indicator tube, the pilot will at least be reassured that he is low and over the edge of the desired landing area, and that all that need be done to effect a reasonably safe landing is to cut his motor and stall in the customary manner.

Although passive reflector antenna networks may operate as either of reflectors ll, l2 or i3 to give suitable landing indications in the above manner it is considered preferable to obviate any chance of false or misleading landing indications being set up by trees, hangers, telephone wires and other reflecting objects by employing a relay circuit in connection with each of the reradiators ll, l2 and i3.

A preferred form of such a reradiator is shown in Fig. 3. In this circuit, a single antenna 5l having preferably directional characteristics is employed both for reception and transmission and is thus directly connected both to a detecting receiver 52 and a transmitter 55. Coupled sections 58, 59 are alternatively operative in a manner which will hereinafter be pointed out to make a single freely conducting path between antenna l and receiver 52 alternately with a single freely conductive path between antenna 5I and transmitter 55. Network 52 supplies for each received wave train a uni-directional impulse corresponding to the wave train envelope and passes the same on for ampliyiication in amplifier 5&3. The impulse is then delayed a certain amount in an appropriate network 5l! whence it is relayed to transmitter 55 to modulate a fresh wave train preferably of the same carrier frequency as that generated by transmitter I9.

The control circuit alternately operating coupled sections 58 and 59 is operative normally to resonate coupled section and to detune coupled section 58 whereby a normal single conductive path is established between antenna 5l and receiver 52. control impulse is supplied to tune coupled section 5S (thus blocking the input line to receiver 52) and at the same time to detune coupled section 59 (and hence effectively open the output line from transmitter 55). Ihe delay instituted by network 5l is of a sufficient 'magnitude to permit the above-mentioned tuning and detuning alternation of coupled sections 5S and 5S 'to take place before a wave train corresponding to the received wave train is supplied from transmitter 535. The control circuit for coupled sections 58, EQ is also effective to maintain this alternate tuned and detuned state for a sufcient period of time to permit transmission of the complete fresh wave train. Shortly thereafter this control circuit returns coupled sections 553 and 59 t6 their normal respective detuned and tuned states awaiting reception of a newA wave train for a repetition of the cycle of operation, as will be clear.

In the form shown, the control circuit for coupled sections 56 and 59 comprises a multivibrator Gil controlled by output of amplifier 55, a clipper 5l, and means supplying output energy from clipper el in two lines, one in opposite phase relation to the other, as by means of an inverter-clipper network S2 included in one of the lines. Operation of this control circuit will be better understood by reference to 'the graphical representations of Fig. Kl, wherein the series of curves a, b, c, d, e, represents progressive voltage conditions in this control circuit, all said voltage conditions being plotted as a function of the same time scale. Curve a of 4 represents the amplified detected envelopes of the successive received wave trains as present in the output of amplifier 5S and as supplied to control multi-vibrator til. Multi-vibrator e@ is preferably of the single stability type wherein a complete cycle of alternate operations is controlled by a single impulsing or triggering condition. Output of multi-vibrator fifi is thus of the form shown in curve b, wherein it will be observed that one alternate operation thereof is always instigated (note leading edges 63) by the control impulses, and the other alternate operation always occurs a relatively small interval of time thereafter (note walls ii) as determined by a fixed charge leaking off a condenser through an appropriate resistance to a point where conductivity of the tube of the multi-vibrator Sii is cut olf. The time-constant determining the remaining portion of the multi-vibrator cycle is relatively great compared with that defining the above-mentioned first portion (from edge 63 to wall S) and is preferably large enough so that application of a succeeding control impulse` is necessary to repeat the cycle of operation.

As indicated above in the discussion of cou-v` Upon detection of a wave train, a

pled sectionsii, a steady control potential is necessary -tomaintain-a coupled section in its tuned state. Accordingly, clipper 6I is provided to clipY out.undesiredportions oi the output signals frcinmulti-vibrator iiii. Curve c represents such-a clipped signal; and, inasmuch as the steady portions of this curve represent the longer intervals of time during which connection between transmitter 55 and antenna 5I is to be locked, this signal may be applied directly to control the tuned. or detuned state o' coupled section 59. Since coupled section tuned for all intervals in which coupled section 51% is detuned, output from clipper need only be clipped and displaced with respect to thezero axis (as by a single discharge tube in network 52a and a battery 62') to supplysuitable control impulses vfor coupled section 53 (see curve d). It is to beV noted in connection with the control potential supplied to coupled sections 58 and 59 that the potential of these signals, when controlling either coupled section for resonance, is alwaysv zero. In this manner, it may beassured that coupled sections and 59 will resonate at the instants at whichthey are intended to and that such resonance will Vbe relatively :independent of varying voltage conditions throughout the-.control-potential generating circuits.

.As a further illustration of preferred timing circuits in the arrangement of Fig. 3, delay network 5i' is operative .to eifect such a delay in the impulses yielded from amplier 53 (see curve a) that fresh wave trains will be transmitted while coupled section 5s is detuned and coupled section 58 is in a timed condition. Curve e graphically illustrates the requisite delay, and it will be noted that all of .the delayed impulses @5 are supplied during these intervals.

In certain cases, it is preferable to modify the reradiator Yof Fig. 3 to include one antenna for transmission and another for reception. rIhe receiving antenna is then non-directional in character so that no matter what the direction of approach of homing aircraft, wave trains transmitted therefrom Yare received with substantially uniform magnitude for any particular distance from the reradiator unit. rIhus the de ired landing path signals as reradiated from the transmitting antennae of units II, I2 and i3 are at a relatively uniform and high level regardless of the direction of approach of an aircrat to the landing area. It is to be noted that in these two-antenna reradiator arrangements, received energy is passed directly to the receiver, there being no conductive path to the transmitter from the receiving antenna and accordingly no need for a coupled-section bloclnng arrangement in the transmitter output line. However, in order to prevent wave trains transmitted by the reradiator from beingdirectly received and reampliiied, a coupled section or other blocking means should be included in the receiver input lines, as will be clear. Furthermore, if desired coupled section 59 may be omitted from the circuit of Fig, 3-provided the transmitter circuit 55 does not absorb too much received energy.

If it is desired that the above-described reradiatcr system (which is normally silent) be used to aid in homing aircraft in addition to establishing a secret glide path in the above-described manner, the antenna 5I of one of reradiator units II, I2 or I 3 (preferably unit I!) is non-directional in character whereby the pilot of the homing aircraft will be able clearly to know the location of the landing area no matter from what direction vhe approaches. If there happenV to, be a number of landing fields in one vicinity, all of which elds have glide-path-dening apparatus of the above-described nature, homing aircraft willbe confronted with a plurality of confusing signals unless something is done to identify each landing area with respect to the others. In such case, the homing reradiator unit of each field should be so conditionedas,positively to identify the field. To this end, a keying unit 56 may be provided to render this reradiator unit alternately effective and ineifective to reradiate wave trains in accordance with akeyingpattern, say, a given Morse-code signal, at va slow enough rate so that the indicating apparatus on the aircraft will show the same by an on-and-oi flash of the indication corresponding to Athat particular reradiator unit. Keyer 6B is connected in the .form shown in Fig. 3 to amplifier 56 to control the biasA of an amplier tube therein, whereby amplifier 56 is rendered alternately effective and ineffective in accordance with the keying signal, as will be clear.Y

In order to demonstrate that the circuit such as that shown in Fig. 3 for each ofthe reradiator units will be responsive to the exclusionof `wave trainsreradiated by others of these reradiator units, three typical ,conditions will be examined. In Fig. 4, .curves f, g, and h are representative of concurrent phenomena in therespective. reradiators I2, i3 and .I iwhenthe aircraft is on course; curves i, 1' and 7c represent corresponding phenomena in reradiators I 2, I3 and II whenthe aircraft is so far to the left ofthe course as to be normal to line joining reradiators Il `and i3 and equidistant from these latter two reradia. torsY (in other Words, substantially normal to the proper course) and curves l, m and n represent thesephenomena .when the aircraft is 180.o 01T course. In the following comparative instances, reradiators i2 and I3 are one4 hundred yards apart and spaced substantially a half mile from thefar reradiator I I the envelope of transmitted wave trains has aneiective width of substantially twomicroseconds; delay throughoutthe receiver, amplifier, 'multi-,vibratonv and clipper stages is two and a half microseconds; delay through receiver, amplifier, delay and transmitter stages is three microseconds, and the time constant of the circuit controlling therst alternateroperation ofmulti-vibrator 6i! is adjusted to Vproduce a time interval between walls 63 and Se equal substantially to 15 microseconds. In .the showings of curves f through n both vthe transmitted wavetrainsand detected envelopes have been shown as square waves of the same magnitude merely for purposes of clarity-the purpose of the curves being solely to demonstrate the relativertiming .ofthe respective reradiator units II, I2, I3.

Under the on course conditionsreradiators I2 and ISwill -be the rst to receive a given wave train. This fact is indicated on curves f, and y, by the concurrent square waves. IIN)`v and IUI. Due to the fact that reradiator IIA is a half mile distant,v the same Wave train vis received there approximately 2.7 ymicroseconds later and is thus indicated as square wave Il'I2 on curve h. Approximately a half. mcrosecond after y,termination of the` received wave trains, the.. coupled sectionj associated with each ofthe reradiators is operated to a tuned state whereby receiver 52 is effectively blocked., Substantially, a half vmicrosecond after such tuning, each of the reradiator units transmits a fresh Wave train as in- Y dicated by square waves two reradiated wave trains reach unit II, the Y coupled section 58 associated with unit II has been tuned, whereby unit II will be non-responsive to such reradiation. Due to the fact that all the coupled sections 58 are tuned for a period of the order of fifteen microseconds, both reradiator units I2 and I3 are similarly non-responsive to the wave train transmitted from unit II, as indicated by square waves |02" on curves f and g occurring prior to the instant at which coupled section 58 is detuned. Thus, under these on course conditions, no one reradiator unit is effective to operate any other, and all the reradiator units are operative independently and exclusively of each other.

Referring to curves i, j, and 7c, the condition of an airplane approaching from the left, normal to the line joining units I I and I 3, and equidistant from these units, will be considered. Under these conditions, reradiator unit I2, being the nearest unit to the aircraft, will be rst to respond to a particular wave train transmitted therefrom. This fact is indicated by square wave |05 on .curve-i. Inasmuch as both reradiator units II and I3 are equidistant from the aircraft, they will be simultaneously responsive to Aa wave train transmitted from the aircraft, as indicated by concurrent square waves |06, and |0"I on curves 7' and 7c respectively. As before, in the case of curves f, y and h, all the coupled sections 58 are tuned approximately a half microsecond after termination of a received wave train, and substantially another half microsecond thereafter each reradiator unit transmits a fresh .wave train, as indicated by square waves I 05', |06' and |01', respectively. Again the coupled-sections 5,8 are maintained in a tuned condition for substantially fifteen microseconds, and it will be noted thatthis period provides ample time to render each one of reradiator units nonresponsive to energy transmitted from any other, as will be clear, from a study of the respective times that these reradiated wave trains pass the respective reradiator units (see waves |05",

Curves Z, m and n illustrate the case of an airplane 180 off course, that is, equidistant from reradiators I2 and I 3 but relatively remote therefrom as compared to unit I I. Inthis case, therefore, reradiator unit II is first responsive to a particular wave train, as indicated by square wave I|I;.and units I2 and I3 are simultaneously responsive to the same wave train approximately 2.7 microseconds later, as indicated by square'waves II2 and I|3 respectively. '.[t will be clear that this sequence of operation of the reradiator units is the inverse of that shown in curves f, .g and Yh and that therefore, the coupled sections 58 associated with each one of theseunits areV always operative to render any one of themnon-responsive to wave trains reradiated from the other unitsfnote waves III, |I2 and |I3 all occurring before coupled sections 58 are returned to their detuned states).

It `will be recalled that indications 48, 49 and 50. in the configuration shown in Fig. 5 are representative of the desired pattern which should be followed by the pilot when landing on instruments :by flying in the plane defined by reflecting antenna elements II, I2 and I3 of Fig. 1. Even under the best conditions these willrnot be the only indications obtained on the screen of the indicator tube. For example, there may always be a certain amount of ground reflection. However, this ground reilection need not cause any confusion with respect to the desired landing configuration inasmuch as the ground will always be the reflecting object nearest the aircraft when a landing is to be effected. Thus the ground may show up as an indication such as 68 in Fig. 5

(well displaced on the distance scale with respect to the glide-path indications). This ground reflection indication provides an additional source of useful information to the pilot inasmuch as he will always have an indication of the absolute altitude of the aircraft and can readily imagine his location with respect to the desired landing area at all times.

A few other possible indicator tube configurations Will now be considered to give an idea of how to practice an instrument landing method in accordance with the invention and also to show that there is no chance of mistaking the correct glide path. In approaching a landing field equipped with reradiators such as I I, I2 and I3, the pilot, knowing the distance from reradiators I2 and I3 to reradiator II, will immediately be able to recognize on the screen of tube 3| the configuration indicative of these reradiators, since on the distance scale, indication 50 would be displaced from indications 48 and 49 a distance corresponding to the distance between reradiator elements I2 and II or I3 and II in the direction of flight. If the symmetric directivity axis of antenna 20' is aimed at reradiator I I and the pilot is flying below the glide plane defined by reradiator elements II, I2 and I3, this fact may be at once evidenced by a configuration such as shown in Fig. 6 wherein the V pattern 69 indicates that reradiating elements I2 and I3 are above his plane of flight. As before in the illustration of Fig. 5, the spot 'I0 indicates the relative position of reradiator II. Both indications 89 and 'I0 taken together would thus convey the information that the pilot is descending on too gradual a glide plane, that is, below the proper glide plane, although he is headed directly for the landing area.

Another indication that may be obtained is shown in Fig. 7. InY accordance with this figure reflections from reradiatorsV I2 and I3 form an inverted V 'II on the screen of the indicator tube. This indication taken in conjunction with the spot V'I2 would indicate that the symmetric directivity axis of antennae 20" is directed at reradiator I I but that this axis is above, rather than in, the desired glide plane. If this axis coincides with the axis of flight, it is clear that under the circumstances, the pilot is heading directly for the landing area and is descending on too steep a glide path. By appropriately maneuvering his plane between the two indications shown in Figs. 6 and 7, they configuration of Fig. 5 will be obtained as a mean and the correct descent thus established. As above indicated, when the pilot passes the edge of the eld and hence passes reradiator elements I2 and I3, the re-iiections due to these elements will disappear from the Vscreen of the indicating tube, and a sole indication 'I3 as shown on the screen of Fig. 8 may be obtained.

This disappearance of reections from reradia-1 tors I2 and I3, as above indicated, will put the pilot on guard that the time is safely appropriate for landing.

It should be noted at this point that, if by any chance there are other aircraft at the airport, perhaps in the desired landing area, this fact will be made appar-ent on the screen of the indicator tube; the pilot will be able to take account of their presence by so maneuvering his lcraft that reflections due to these other aircraft do not form spots but rather dagger shaped indications on the screen of his indicator tubethereby assuring himself that he is not headed directly at these other aircraft, In any case, if amplifying means is provided the reradiator systems, as shown in Fig. 3, a relatively strong indication of the proper glide path will always be given on the screen of the tube, and the presence of other aircraft in the way may be noted by indications which are relatively weak in coinparison with the strong glide-path indications.

A further typical indication that may be obtained with the apparatus described is shown in Fig. 9 This indication, it will be observed, is very similar to that shown in Fig. 6. However, it could not be confused itherewith inasmuch as the spot l indicates reflecting antenna unit l l to be closer to the aircraft than units l2 and S3 (see indications l@ and It, respectively). Thus, the pilot would know that he was approaching the field from the wrong direction and that he must make a half circle about the field. As he begins t0 bank, units il, l2 and lli may appear on his screen in the configuration shown in Figl0, wherein indication '58', being weaker than indication 'it and also displaced further along the distance scale, indicates reradiator l2 to be nearer the aircraft and the axis of antennae iii to be to the left of the true glide path. inasmuch as both these indications 'is IS' appear horizontal, it would seein that the pilot is in a gentle bank wherein the plane of his wings lies in the plane of the desired glide path, but he is not yet headed in a correct direction for landing, although the axis of antennae lili is directed at reradiator ll.

In accordance with another preferred embodiment of my invention, a correct and safe glide path is established with reradiators l2 and i3 disposed on the ground in the saine manner as reradiator li. Assuming :this condition, the correct glide path would be recognized by a pilot if he so Orients the axis 0f antennae 2Q' that it is directed to produce an indication such as shown in Fig. ll on the screen of his tube El. This indication comprises an inverted V 'i9 and a spot 3S. The invented V indicates reradiators i12 and I3 to be symmetrically disposed about the line of flight and to be below the plane of nightthe correct condition precedent to this alternate method of landing. In employing this type of system, it is preferred that lines Sl and Sl be ruled or otherwise superimposed on the screen of the indicator tube to indicate the appropriate angularity of inverted 'is for the proper glide angle, as will be clear.

It is to be noted that the detecting apparatus described in connection with Fig. 2 has the inherent disadvantage of being unable to discriminate between two reflecting objects having the same reflecting characteristics and spaced equally from the pla-ne symmetrically about its axis of flight. Under such conditions, it is clear that a cancellation of direction indicative deilection potentials may cause a mere spot to appear on the screen of indicating tube 3l, whereas in truth there are two reflecting objects. In descending by means of the above-described glide-path, this inability to discriminate between two equidistant objects symmetrical about the axis of ight may be a cause lfor considerable confusion to the pilot of the aircraft as he is trying to land. In such case, this confusion may be entirely obviated by providing suit-able keying means QQ (see Fig. 1) for alternately rendering units l2 and lll effective to reradiate received wave trains, If this alternation is performed at a rate greater than the persistence of vision on the screen of the indieating apparatus on the aircraft, it is clear that both reradiators l2 and i3 will appear to be simultaneously eective. Keying means suitable for this purpose is Kconnected to control the bias on amplifier 553 for units l2 yand i3 in the same manner as that above described in connection with keyer S.

Any possibility of confusing signals reradiated from units i2 and I3 may also be avoided in accordance with a further embodiment of the invention by controlling the delay network 5l in one of these reradiator units effectively to impose a delay in the wave trains detected thereby greater than the delay imposed by the corresponding network 5l of the other reradiator unit. In such case, the aircraft will obtain a set of indications such as shown in Fig. 12 for a condition of proper glide path ight. As in the illustrations above-described in connection with Figs. 5 through 10, the pilot would still strive to maintain indications for reflections from unit ll as a spot lil and to keep the other indications $3, t5' indicative respectively of reflections of units l2 and i3, in a horizontal although not aligned disposition. In this latter case, the pilot must depend solely upon equality cf deflection for indications 68 and 63 to keep himself upon the course, whereas in the abovedescribed examples of Figs. 5 through 10, any noticeable offsetting of one of the indications t8 with respect to the other 49, along the distance scale, is a further indication of a deviation from the correct course (see also Fig. 10).

There are several reasons for including in the reradiator unit of Fig. 3 means for effectively delaying detected wave trains. First, by delaying the received impulse prior to ree-transmission, it is possible to re-transmit at a relatively high level compared to the received signal strength, without any danger of occurrence of singing Furthermore, because of the delay, aircraft landing by means of the reradiated signals may fly as close as the immediate vicinity of the reladiator without there being any danger of the blanking impulse generated by network i8 in the aircraft receiver cutting out Wave trains reradiated from a particular reradiator unit, as will be clear.

For a better understanding of the operation of the invention, the above discussion has considered the antenna unit 2d' on the aircraft as fixed and having its directional axis of symmetry coincidental with the axis of flight of the aircraft. In anY alternate preferred operation in accordance with the invention, however, and at very short wavelengths, all the antennae 2t are mounted as a unit having the above-donned directional characteristics with respect to the axis of disposition of the unit. The unit is orientable and to this end preferably mounted on a ball and socket swivel joint or the equivalent, whereby it may be manually aimed by the pilot at reradiator unit ll. In this manner, the indication corresponding to the unit ll is always maintained Vgenerally as Il I 15 as a spot, in order better to identify the landing iield and better to be able to picture landingr conditions. Once the pilot has in this way so maneuvered his craft that its axis of ilight is directed at reradiator II and is in the correct glide path, the swivel mounting for antenna 2li should be locked in position so that the p-ilot will have one less operation' to think about.

In a further embodiment, analogous to that described above in connection with Fig, il, an aircraft may be guided to land toward the near end of the field, whereby a longer and safer taxiing distance is permitted before braking and stopping. In this latter embodiment, all reradiator units II, I2 and I3 are disposed on the ground, unit I I preferably being in a pit covered with a non-shielding material and located in the middle o the landing area 50 that the same pit may be used no matter what the direction of wind, and hence direction of landing. As shown in Fig. 13, the screen of the pilots indicator tube r3l is ruled or has otherwise superimposed thereon a system of guide lines 8l, 8l', S'I" and 81 indicating certain slopes for indications 86 and 86 at different distances of approach therefrom (corresponding to reflections from reradiators I2 and I3). For different distances from the landing area, the pilot still aims his aircraft at the reradiator unit Il in the middle of the field; but it Vwill be noted that as he approaches the field, the slope of indications Sii and 86 must be flattened in accordance with the guide lines Si', Bl and finally 81V" to bring him down properly to the near edge of the landing area, as will be clear. A particular advantage of this latter arrangement is seen in the fact that the three indications 85, BIS and Se will always be obtained and clearly visible to the pilot up to the time his wheels touch the ground, whereas in the other above-described forms, as illustrated in Fig. 8, indications on the cathode ray screen corresponding to reradiation from units I2 and I3 disappear before the wheels touch the ground.

Where economy and extreme portability are prime considerations, a much simplied form of reradiator unit may be employed, that is, much simpler than the form shown in Fig. 3. This simplified reradiator is shown in Fig. 3U. and comprises a single antennar 96 connected directly to one corner of a conjugate network, designated Input and output connections of a carrier-frequency amplifier 92, responsive Vto the carrier frequency of wave trains transmitted from the aircraft, are connected respectively to conjugate corners of network QI. The remaining corner of conjugate network Si includes a balancing network 93 so that there will be no interchange of energy between input and output circuitsof amplifier 92 due to their connection to network 9|, as will be'clear, Suitable delay as instituted by network 94 in the output of amplier 92 makes it possible for pilots alwaysy to see indications corresponding to reradiated wave trains from this apparatus no matter how close they approach it, as explained above in connectionwith. Fig. 3. In order to prevent one such reradiation unit from picking up and relaying wave-trains already reradiated from other units a square-wave generator 95 supplies control signals of suitable duration to keep a coupled section SI (in the ampliiier-input circuit) tuned during reradiation and for a sufficient length of time to block out reception of energy reradiated from the reradiator units. This type reradiator unit has'the inherent disadvantage of dissipating i sponding energy in network 93 and is therefore undesirable for high power. However, when there are few reecting bodies in the neighborhood of the landing area, as when a seaplane is landing on a water surface, wave trains reradiated from a low power unit such as the arrangement of Fig. 3a will be clearly identiable, and a landing may be effected with relative safety, inasmuch as there is no need to worry about over-running the landing area.

In certain cases, in connection with instrulanding on water surfaces, where there substantially no radiant energy reflecting bodies except the water, it is preferable to use passive reectors as the reradiator units, inasmuch as such reflectors may be mounted on buoys and would require little or no attention or upkeep other than to be towed into the appropriate positions depending upon the direction of wind. The arrangement illustrated in Fig. 1 may be considered as an illustration of this system. The landing area then constitutes a water surface.

The systems described provide relatively simple methods for enabling aircraft to be landed under conditions of poor visibility; and, as the aircraft equipment is concerned, no substantial additional apparatus need be provided 'to follow out the method, provided the aircraft is equipped with obstacle detection or radio-location equipment of substantially the above-indicated nature. lt will be clear that in ition to uses ofV this single piece of apparatus on the aircraft :tor both instrument landing and locating aircraft, it may be employed to give a clear indication of absolute altitude and also in certain radio course navigation cases. 1n the latter instances reradiator units of the nature in 'floated for instrument landing applications y be used that is, single reradiators, such as shown in Fig. 3 or 3a, and suitably spaced along a desired course, will give pilots a steady c ourse if they strive to maintain indications Correto reradiation therefrom as single spots, as will be clear. For instrumen'J landing purposes, the only apparatus that need be .installed in the vicinity of the landing area may be so compact and simple, as to be readily portable on light trucks, or even passenger automobiles or trailers. The apparatus :installed in the vicinity of the desired landing area is .furthermore so simple as to require relatively little eX- pert care or supervision. The landing arca installation is so readily portable that a new field or a new Ycourse may be dened in an above-indicate'i manner in the space of Vrelatively few minutes.

By making receivers associated with the reradiators il, IE and I3 highly discriminatory as to the carrier frequency to which they are to respo'l namely, the carrier frequency :tor transmitting wave trains from the aircraft, secrecy of the apparatus may be still further enhanced. it :is to be noted that inV order for aircraft to try to locate such a landing area defined in 'the above-indicated manner, the enemy must under the circumstances know precisely'the carrier frequency to which the reradiator elements will respond. Even so, he will give his presence away by transmitting a nurnber of wave trains himself in an effort to :dnd

any enemy Y delays and recognize them in interpreting his image upon the cathode ray screen; and, clearly the chances of an enemy pilot knowing of these delays, or, in that event, how to take account of them, are relatively slight. If any enemy were to try to land by assuming wrong delays in connection with reradiators II, I2 and I3, he would land either ahead of or beyond the desired landing area at -too great a speed and hence would very likely crash.

Conceivably, it may not be possible to keep the character of the reilector units secret from enemy spy agents, and it might therefore be possible for 'them to set up apparatus similar to reradiators II, I2 and I3 in the neighborhood of the landing area so as to give false glide-path indications and thus to jam the pattern of signals reradiated by elements Il, I2 and I3. This undesirable feature can be met, however, by relatively simple precautions. For example, keying means 66 may be provided in connection with each one of the reradiator elements so as alternately to render each of these units effective and ineffective to reradiate received wave trains. The frequency of this alternation would preferably be at some visibly recognizable rate whereby the pilot of a friendly aircraft could recognize his airport from a blinking on-andoff of the three indications representative of units il, I2 and I3 in accordance with a known keying signal.

As above indicated, it is preferable that impulse generator I3 carried on the aircraft be allowed to run free and thus to generate intermittent impulses (that is, non-periodically recurrent). Now, if several aircraft are simultaneously approaching the desired landing area, each, if equipped with such an :impulsel generator, will be transmitting unsynchronized wave trains having no regular recurrent pattern and hence relatively few instants of concurrence. Each airplanes saw-tooth generator is also, as above noted, synchronized with its impulse generator I8 so as to produce a fresh saw-tooth for each impulse generated. Thus, each airplane will show on its indicator tube 3l steady indications corresponding only to reflections or reradiations from units II, I2 and I 3 for wave trains transmitted directly from the particular aircraft. Reradiation from units II, I2 and I3 corresponding to wave trains transmitted from other aircraft, the latter wave trains being unsynchronized with that from the said particular aircraft, will, accordingly, not appear as steady indications on the screen of the indicator tube 3l. Thus, with a random type impulse generator, it is possible for a number of aircraft simultaneously to converge on the landing area. Standard interplane communication means will serve to aid the respective aircraft in notifying each other when they are about to land so as to avoid collision.

It is to be noted that the invention has particular utility in defining the landing deck of an aircraft carrier, as when bombing and torpedo operations are carried out under smoke screen or night conditions. In such case reradiators I2 and I3 should be disposed at the approach end of the deck and spaced apart the width of the deck. Reradiator I I again should be in a pit on the deck; or, in order to be more certain of staying on the deck after passing units I2 and I3, two further units spaced similar to the disposition of units l2' and I3 should be employed between unit Il and units I2 and I3 and in the glide plane. In the latter manner, even after passing the outer markings, as defined by reradiation from units I2 and I3, the pilot will still have bearings on two side markers and the spot from unit II to help him keep his course. The fact that units I2 and I3 cannot be spaced much more than the width of the ship means that angular changes in indications 49, 50 will be more readily apparent for up or down deviations from the glide plane. Accordingly, a clearly dei-ined glide path may be set up for such use. There will be no danger of colliding with units I2 or I3 on landing since the pilot should always, as indicated, maintain reradiation from these units on opposed sides of his axis of flight; and, as he approaches these units, his relative proximity to one with respect to the other will become increasingly noticeable, it thus becomes increasingly simple for him to pass between units I2 and I3, as the landing deck is approached. In this connection, it is further to be noted that, since an aircraft carrier is always headed correctly with respect to the wind whenever landing or take-olf operations are proceeding, the reradiator units may be rigidly and permanently installed.

It is clear that although I have described my invention as being particularly adaptable to obstacle-detection systems of the pulse modulation type, it may be equally or more desirable to employ other systems. For example, the Varying frequency system, wherein received reflected waves are caused to beat with those transmitted to give a distance indication, may be adapted to directional-discriminating purposes by using appropriately directive antennae, as will be clear. In such case, there would be no need to introduce the delays and blanking signals described above as being desirable in a pulse system, for conjugate networks may be employed to discriminate concurrent signals. It is to be noted that with this latter type system wherein no delays are necessary, the reradiator units may be substantially simplified and reradiation therefrom will never be unobservable on the aircraft, no matter how close it approaches a reradiating unit. Singing between the respective reradiator units may be prevented by appropriately interconnecting them by conjugate network means, whereby energy reradiated by one unit will not cause operation of another. Furthermore, ground reflection (absolute altitude) having no delay, and reradiation from units II, I2, i3, having substantially no delay, will appear properly related on the same time or distance scale on the aircrafts indicating apparatus, thus giving a pilot a more readily interpretable read- 1ng.

While the invention has been described in particular detail in connection with the preferred forms shown, it is to be understood that many modifications, additions, omissions and adaptations may be made without departing from the spirit `and scope thereof as delined'in the appended claims.

What I claim is:

1. The method of landing an aircraft on a desired area in the vicinity of which a plurality of relatively widely spaced fixed radiant energy reflectors are disposed, said aircraft being equipped with radiant energy radiating and receiving apparatus for detecting the general direction of an object and indicating the same on the screen of a cathode ray oscillograph, which method comprises transmitting energy from said radiating apparatus, receiving said radiated energy after reflection from saidv reflectors, orienting the aircraft to such a position that images thereby formed on the oscillograph screen form a predetermined pattern, said .pattern being understood as indicative of received reflections from said plurality of reflectors, and continuing flight while maintaining substantially the same pattern on the oscillograph screen.

2. An antenna system for aiding in the instrument landing of aircraft along a desired glide path which comprises a plurality of reradiating antenna units responsive to substantially a given wave-length and well displaced from each other about the area in which the aircraft is to be landed, one of said units being placed in a plane including said desired glide path, and two of said units being equally spaced apart on opposite sides of said plane, each of said antenna units being displaced with respect to the others of said plurality a large'number of said wavelengths.

3. An antenna system `according to claim 2 wherein said reradiating antenna units include relay means, whereby energy detected by said relay means may be reradiated at a higher signal level.

4. An antenna system according to claim 2, wherein said reradiating antenna units are generally directional in a sense longitudinal with respect to the desired glide path and opposed to the direction of flight in landing.

5. An antenna system for aiding in the instrument landing of aircraft, said system comprising a radiant energy reradiating unit distant from a desired landing location and in the desired direction of landing, and two further reradiating units in front of the desired landing location, each of said further reradiating units being disposed on one side of the desired glide path.

6. An antenna system according to claim 5,

means and reradiated by said reradiating means, said detector means including means detecting the respectivedirections of such reradiation with on the correct glide path.

wherein said further reradiating units are elevated above the level of the landing location and said first-mentioned reradiating unit is substantiallvy at the level of the landing location, whereby all three of said reradiating units lie in one plane at an angle to the plane of the landing location.

7. The method of orienting an aircraft in a desired glide path, which method comprises transmitting brief trains of radiant energy from the aircraft, reradiatng Said energy from at least two displaced points in the vicinity of a desired landing location, detecting the respective delays from transmission to reception of energy so transmitted from the aircraft and reradiated from said points, detecting the respective directions of reradiation from said points with respect to the aircraft, visually indicating on the aircraft the direction and relative distance of said points with respect to the aircraft, orienting the aircraft to such a position that a predetermined and understood visual image is 10. An antenna system according to claim 5, wherein all said reradiating units are disposed at substantially the level of the desired landing area.

11. An antenna system for aiding in the instrument landing `along a desired glide path of aircraft equipped with apparatus for transmitting on a carrier brief wave trains of said carrier, said system comprising a plurality or passive-network reecting antenna units tuned to substantially the frequency of said carrier and displaced from each other about the area in which the aircraft is to be landed, one of said units being placed in a plane including said desired glide path, and two of said units being equally spaced apart on opposite sides of said plane, each of said antenna units being displaced'a large number of Wave lengths at said carrier with respect to others of said plurality.

12,'A system for aiding in the blind landing of aircraft on a Water surface along a desired glide path, said aircraft being equipped withY .on opposite sides of said plane, reradiating means mounted on each of said buoys, each of said buoys being displaced with respect to the others of said plurality a large number of said wavelengths.

13. An antenna system according to claim 2,Y

further comprising keying means coupled to one Vof said reradiating antenna units for rendering said one antenna unit eifective to reradiate energy at one level alternately with'another level, said keying means being operative at a visuallyy resolvable frequency. l y

ROY C. HOPGOOD, JR.

REFERENCES YCITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Germany Aug. 20, 1932 

